Half-Cell Potential Analysis of an Ammonia Sensor with the Electrochemical Cell Au | YSZ | Au, V2O5-WO3-TiO2

Abstract

Half-cell potentials of the electrochemical cell Au, VWT | YSZ | Au are analyzed in dependence on oxygen and ammonia concentration at 550 °C. One of the gold electrodes is covered with a porous SCR catalyst, vanadia-tungstenia-titania (VWT). The cell is utilized as a potentiometric ammonia gas sensor and provides a semi-logarithmic characteristic curve with a high NH3 sensitivity and selectivity. The analyses of the Au | YSZ and Au, VWT | YSZ half-cells are conducted to describe the non-equilibrium behavior of the sensor device in light of mixed potential theory. Both electrode potentials provide a dependency on the NH3 concentration, whereby VWT, Au | YSZ shows a stronger effect which increases with increasing VWT coverage. The potential shifts in the anodic direction confirm the formation of mixed potentials at both electrodes resulting from electrochemical reactions of O2 and NH3 at the three-phase boundary. Polarization curves indicate Butler-Volmer-type kinetics. Modified polarization curves of the VWT covered electrode show an enhanced anodic reaction and an almost unaltered cathodic reaction. The NH3 dependency is dominated by the VWT coverage and it turns out that the catalytic properties of the VWT thick film are responsible for the electrode potential shift.

A review of mixed-potential type zirconia-based gas sensors

“…In order to monitor the concentration of harmful slip NH 3 , the development of a highly sensitive, selective, and stable NH 3 sensor is in a strong demand in recent years. Table 5 summarizes the typical examples of mixed-potential type YSZ-based NH 3 sensors which have been developed recently in order to satisfy these requirements and needs [57,62,121,[166][167][168][169][170][171][172][173].…”

“…Kida Fig. 24 [57,166]. VWT is well-known as an effective SCR catalyst [174] and it is believed to produce activated NH 3 species, resulting in an increase in electrochemical activity of SE against NH 3 .…”

“…A strong acid-base interaction between SiO 2 and NH 3 was postulated to contribute for such highly selective NH 3 detection. Figure 27 summarizes and compares the NH 3 sensitivities for the several above-mentioned YSZ-based sensors [57,62,121,[166][167][168][169][170][171][172][173]. The highest NH 3 sensitivity was obtained by using the YSZ-based sensor using MoO 3 -SE operating at 500°C; the sensitivity (Δemf) was still as high as 30 mV even to 10 ppm NH 3 [167].…”

Atomic-Scale Observation of Migration and Coalescence of Au Nanoclusters on YSZ Surface by Aberration-Corrected STEM

“…Here, we apply aberration-corrected scanning TEM (STEM) to directly observe surface migration, coalescence, and atomic rearrangement of Au clusters on an Y:ZrO 2 (YSZ) (100) support at the atomic scale. The heterogeneous Au/YSZ system is chosen because it finds a plethora of applications as gas sensors 16 17 , gas pumps 18 , solid oxide fuel cells 19 20 , and electrochemical reactors 21 . We offer definitive evidence that the Au/YSZ heterointerface can be initially epitaxial and coherent, and demonstrate that it undergoes structural rearrangement by migration and coalescence, which finally induces transformation of Au particles and also breaks coherency of the heterointerface.…”

A mixed potential based sensor that measures directly catalyst conversion—A novel approach for catalyst on-board diagnostics

“…A special stainless steel setup comprised two separated chambers and a gas inlet on each side (details in [42]). For gas tight sealing, a mica seal, cut to the right geometry by a laser, was used.…”